$ C = \left[\begin{array}{rr}3 & 4 \\ 5 & 2\end{array}\right]$ $ E = \left[\begin{array}{rrr}4 & 3 & -2 \\ 0 & 2 & 0\end{array}\right]$ What is $ C E$ ?
Explanation: Because $ C$ has dimensions $(2\times2)$ and $ E$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ C E = \left[\begin{array}{rr}{3} & {4} \\ {5} & {2}\end{array}\right] \left[\begin{array}{rrr}{4} & \color{#DF0030}{3} & \color{#9D38BD}{-2} \\ {0} & \color{#DF0030}{2} & \color{#9D38BD}{0}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ C$ , with the corresponding elements in column $j$ of the second matrix, $ E$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ C$ with the first element in ${\text{column }1}$ of $ E$ , then multiply the second element in ${\text{row }1}$ of $ C$ with the second element in ${\text{column }1}$ of $ E$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{3}\cdot{4}+{4}\cdot{0} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ C$ with the corresponding elements in ${\text{column }1}$ of $ E$ and add the products together. $ \left[\begin{array}{rrr}{3}\cdot{4}+{4}\cdot{0} & ? & ? \\ {5}\cdot{4}+{2}\cdot{0} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ C$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ E$ and add the products together. $ \left[\begin{array}{rrr}{3}\cdot{4}+{4}\cdot{0} & {3}\cdot\color{#DF0030}{3}+{4}\cdot\color{#DF0030}{2} & ? \\ {5}\cdot{4}+{2}\cdot{0} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{3}\cdot{4}+{4}\cdot{0} & {3}\cdot\color{#DF0030}{3}+{4}\cdot\color{#DF0030}{2} & {3}\cdot\color{#9D38BD}{-2}+{4}\cdot\color{#9D38BD}{0} \\ {5}\cdot{4}+{2}\cdot{0} & {5}\cdot\color{#DF0030}{3}+{2}\cdot\color{#DF0030}{2} & {5}\cdot\color{#9D38BD}{-2}+{2}\cdot\color{#9D38BD}{0}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}12 & 17 & -6 \\ 20 & 19 & -10\end{array}\right] $